[EMAIL PROTECTED] wrote: > The python code below is adapted from a Haskell program written by > Tomasz > Wielonka on the comp.lang.functional group. It's more verbose than his > since I wanted to make sure I got it right. > > http://groups.google.com/group/comp.lang.functional/browse_frm/thread... > > Does anyone know how to turn it into a module (or whatever it's called) > so that I can put it in a > loop and not have to generate the whole list? I've already tried > without success.
I know next to nothing about Haskell, but I believe that it evaluates sequences lazily by default. Python can certainly do lazy evaluation, but you'll have to ask for it explicitly. > > The program solves the following problem: generate the subset X of the > cartesian product S^n that excludes n-tuples defined by wildcards. For > example, if I want to exclude from [1,2,3]^3 the wc's [1,"*",2] and > ["*",3,"*",3,"*"], where "*" stands for a sequence of zero or more > elements of [1,2,3], then I just type > [I think your example and test case may have become mixed up] ... Here's an iterative "lazy" solution. While we're at it, why not use re for pattern matching, rather than rolling our own matcher (this does restrict the set members to characters, maybe that's OK): import re def cp_excl_wc(alpha, N, *exclusions): RE0, RE1 = compile_re(exclusions) def add_one(outer, RE_PATTS): if RE_PATTS: for partial in outer: for a in alpha: next = partial + a if not RE_PATTS.match(next): yield next else: # if there's no pattern to match at this length # don't waste time trying for partial in outer: for a in alpha: yield partial+a cpgen = "", # RE0 holds the patterns that match at the beginning, so are tested # against all iterations less than full length for n in range(N-1): cpgen = add_one(cpgen, RE0) # For the last place, test the full-length patterns cpgen = add_one(cpgen, RE1) return cpgen def compile_re(exclusions): re0 = [] # patterns that can be matched anywhere re1 = [] # patterns that match only the full-length for pattern in exclusions: pattern = pattern.replace("*", ".*") if pattern.endswith(".*"): re0.append(pattern) re1.append(pattern+"$") re0 = re0 and re.compile("|".join(re0)) or None re1 = re1 and re.compile("|".join(re1)) or None return re0, re1 >>> nm = cp_excl_wc("123",3,"1*2","3*1") >>> nm.next() '111' >>> nm.next() '113' >>> nm.next() '121' >>> nm.next() '123' >>> list(nm) ['131', '133', '211', '212', '213', '221', '222', '223', '231', '232', '233', '312', '313', '322', '323', '332', '333'] >>> Is this the sort of thing you had in mind? Michael -- http://mail.python.org/mailman/listinfo/python-list